This invention relates to novel blends of poly(phenylene oxide), liquid crystalline polymers, and non-conductive fillers, and in particular, to such blends having improved combinations of high arc tracking-index, good toughness and good cosmetic appearance.
Poly(phenylene oxide (PPO) and liquid crystalline polymers (LCP) are individually useful as molding resins for general purpose uses, and more specifically in the electrical and electronics industries due to their thermal stability, chemical resistance, and other desirable properties. However, each has limitations for certain electrical applications.
Poly(phenylene oxide) exhibits good impact resistance, is relatively inexpensive and has a high glass transition temperature which gives it good high temperature stiffness properties. The PPO is very viscous and fails to process well in typical thermoplastic fabrication equipment. Previous innovations to improve the processibility relied upon blending the PPO with other polymers, usually containing polystyrene or copolymers with a styrene derived component. These innovations have been very successful at making widely useful PPO based materials (blends) that process very well (for example General Electric Plastic""s Noryl(copyright) product line). However, the PPO blends with polystyrene and similar copolymers have reduced glass transition temperatures and the resulting high temperature stiffness is lower than that of pure PPO by about 40xc2x0 C. or more. There are needs for a material with PPO""s high temperature stiffness that processes well yet retains good toughness.
Liquid crystalline polymers typically process very well and have good high temperature stiffness and good electric properties but are relatively expensive. Blends of PPO with LCP, particularly in combination with fillers for improved arc tracking-index, are potentially advantageous. However, it is commonly known that LCP""s are generally incompatible with PPO, with or without fillers added.
U.S. Pat. No. 5,182,334 discloses that LCP/PPO blends tend to have large regions or domains of the individual polymers rather than fine, well-dispersed domains; large domains tend to produce poor properties. The LCP/PPO blends are said to be compatibilized by reacting the acid or ester functionality on the LCP with a hydroxyl on PPO in an acid catalyzed process.
U.S. Pat. No. 5,006,403 discloses a polymer composite of about 5-97.5 wt. % PPO and about 95-2.5 wt. % LCP, with the two components being present as separate phases in blends. The product blend was a self-reinforced polymer composite, in which the PPO is the matrix, and the wholly aromatic polyester is in the form of predominantly uni-directionally oriented continuous fibers or strands, oriented in the direction of extrusion. The self-reinforced composites are deemed to be particularly suitable for automotive and aerospace applications as replacements for composite components produced by sheet molding compound technology. The increase in tensile strength demonstrated by these fiber self-reinforced composites is offset by the poor elongation properties (a measure of toughness) inherent with this type of morphology (due to the presence of relatively large continuous fibers or strands in the composites).
PPO/LCP blends should exhibit good tracking properties for many electrical and electronic applications. Arc-tracking is a phenomenon associated with the formation of permanent and progressive conducting paths on the surface of materials by the combined effects of an electrical field and external surface pollution. Electrical tracking can occur when a damaged energized electrical part becomes wet, e.g., from electrolytes or condensation. This tracking may lead to flash over and arcing that causes further damage in the electrical part, causing a catastrophic cascade failure. Tracking can occur at low voltages, e.g., 100V AC or less but becomes less likely as the voltage is reduced. The comparative tracking index (CTI) rating provides a quantitative indication of a composition""s ability to perform as an electrical insulating material under wet and/or contaminated conditions. In determining the CTI rating of a resin composition, two electrodes are placed on a molded test specimen. A voltage differential is then established between the electrodes while an aqueous ammonium chloride solution is slowly dripped on the test specimen. The CTI rating of the test specimen is the maximum electrode voltage differential at which, in five consecutive tests, more than 50 drops of the solution must be applied to the test specimen in order to cause tracking to occur. Hence, the CTI value is the voltage at which a molding is found to exhibit conductivity. For some applications in the electrical and electronics industry, the CTI value is expected to be at least 220 volts.
U.S. Pat. No. 4,043,971 discloses a thermoplastic polybutylene terephthalate molding composition containing about 5 to 60 wt. % calcium sulfate and/or barium sulfate to increase its tracking resistance. However, this patent does not disclose or suggest using calcium sulfate in LCP/PPO blends.
A need continues to exist for practical and less expensive compositions and methods for making PPO/LCP blend compositions having improved properties. In particular, a need exists for practical PPO/LCP blend compositions displaying good tracking index properties as well as other desirable characteristics of PPO/LCP blends, and comprising less expensive constituents and fillers.
Our invention includes polymer blends comprising (a) about 50 to 95 weight percent, based on the total weight of components (a) and (b), of poly(phenylene oxide); (b) about 5 to 50 weight percent, based on total weight of components (a) and (b), of thermotropic liquid crystalline polymer; and (c) non-conductive filler, in an amount sufficient to increase the comparative tracking index (CTI) rating of the polymer blend to above 220 volts.
Also included are molded articles comprising polymer blends of our invention, including applications in electronic and electrical apparatus.
Other aspects and embodiments of our invention will be better understood in view of the following detailed description of preferred embodiments and the accompanying figures.